It is known to be of interest in many applications, for example in motor vehicle constructions, to use double-wall tubes between the walls of which may optionally be disposed a filler material in continuous or discontinuous contact with the two walls between which it is disposed. The presence of these two walls and a gap therebetween, which is or is not filled, permits the obtainment of improved properties of insulation from the thermal point of view or the sound point of view. Such tubes may for example be employed for forming exhaust pipes of motor vehicles.
A method for producing such tubes may comprise forming a composite assembly obtained by superimposing two metal sheets maintained at a certain distance from each other by spacer means in one integral piece with one or the other of these two sheets or connected thereto, or separated by a continuous or discontinuous inserted material which may be formed by a metal lattice or netting or by a material in the form of a foam, powder or other material. This composite assembly is then rolled so as to form a tube and the two adjacent ends are assembled by welding. However, a serious drawback of this method resides in the fact that, in the zone of the weld, the two confronting edges of the composite material are formed by sharp edges of the two sheets constituting the inner and outer walls of the tube, so that the weld between these two confronting portions is liable to be of poor quality and the strength and the fluidtightness of the tube may consequently be seriously affected. This is due to inevitable defects in the geometry of the confronting edges and to a defect of homogeneity which is still further increased if a meltable material is located between the two sheets and is mixed with the molten bath during the welding operation.